Method of depositing an optical quality silica film by PECVD

ABSTRACT

A method is disclosed for depositing an optical quality silica film on a wafer by PECVD. The flows rates for a raw material gas, an oxidation gas, a carrier gas, and a dopant gas are first set at predetermined levels. The total deposition pressure is set at a predetermined level. The deposited film is then subjected to a post deposition heat treatment at a temperature selected to optimize the mechanical properties without affecting the optical properties. Finally, the observed FTIR characteristics of the deposited film are monitored to produce a film having the desired optical and mechanical properties. This technique permits the production of high quality optical films with reduced stress.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to the manufacture of high quality opticalfilms, and in particular to a method of depositing an optical qualitysilica film by PECVD. The invention can be applied to the manufacture ofphotonic devices, for example, Mux/Demux devices for use fiber opticcommunications.

[0003] 2. Description of Related Art

[0004] The manufacture of integrated optical devices, such as opticalMultiplexers (Mux) and Demultiplexers (Dmux) requires the fabrication ofoptical quality elements, such as waveguides and gratings highlytransparent in the 1.30 μm and 1.55 μm optical bands. These silica-basedoptical elements are basically composed of three layers: buffer, coreand cladding. For reasons of simplicity, the buffer and cladding layersare typically of the same composition and refractive index. In order toconfine the 1.55 μm (and/or 1.30 μm) wavelength laser beam, the coremust have a higher refractive index than the buffer and cladding layers.The required refractive index difference is referred to as the ‘delta-n’and is one of the most important characteristics of these silica-basedoptical elements.

[0005] It is very difficult to fabricate transparent silica-basedoptical elements in the 1.55 μm wavelength (and/or 1.30 wavelength)optical region while maintaining a suitable difference delta-n andpreventing stress-induced mechanical and problems. Our co-pending U.S.patent application Ser. No. 09/799,491 filed on Mar. 7, 2000 entitled‘Method of Making a Functional Device with Deposited Layers subject toHigh Temperature Anneal” describes an improved Plasma Enhanced ChemicalVapour Deposition technique for these silica-based elements which allowsthe attainment of the required ‘delta-n’ while eliminating theundesirable residual Si:N—H oscillators (observed as a FTIR peakcentered at 3380 cm⁻¹ whose 2^(nd) harmonics could cause an opticalabsorption between 1.445 and 1.515 μm), SiN—H oscillators (centered at3420 cm⁻¹ whose 2^(nd) harmonics could cause an optical absorptionbetween 1.445 and 1.479 μm) and SiO—H oscillators (centered at 3510 cm⁻¹and whose 2^(nd) harmonics could cause an optical absorption between1.408 and 1.441 μm) after a high temperature thermal treatment in anitrogen ambient, typically at 800° C.

[0006] With such a high temperature thermal treatment are associatedsome residual stress-induced mechanical problems of deep-etched opticalelements (mechanical movement of the side-walls) and some residualstress-induced mechanical problems at the buffer/core interface or atthe core/cladding interface (micro-structural defects, micro-voiding andseparation).

[0007] Recently published literature reveals various PECVD approaches toobtain these high performance optically transparent silica-based opticalelements: Valette S., New integrated optical multiplexer-demultiplexerrealized on silicon substrate, ECIO '87, 145, 1987; Grand G., Low-lossPECVD silica channel waveguides for optical communications, Electron.Lett., 26 (25), 2135, 1990; Bruno F., Plasma-enhanced chemical vapordeposition of low-loss SiON optical waveguides at 1.5-μm wavelength,Applied Optics, 30 (31), 4560, 1991; Kapser K., Rapid deposition ofhigh-quality silicon-oxinitride waveguides, IEEE Trans. Photonics Tech.Lett., 5 (12), 1991; Lai Q., Simple technologies for fabrication oflow-loss silica waveguides, Elec. Lett., 28 (11), 1000, 1992; Lai Q.,Formation of optical slab waveguides using thermal oxidation of SiOx,Elec. Lett., 29 (8), 714, 1993; Liu K., Hybrid optoelectronic digitallytunable receiver, SPIE, Vol 2402, 104, 1995; Tu Y., Single-modeSiON/SiO2/Si optical waveguides prepared by plasma-enhanced Chemicalvapor deposition, Fiber and integrated optics, 14, 133, 1995; HoffmannM., Low temperature, nitrogen doped waveguides on silicon with smallcore dimensions fabricated by PECVD/RIE, ECIO'95, 299, 1995; BazylenkoM., Pure and fluorine-doped silica films deposited in a hollow cathodereactor for integrated optic applications, J. Vac. Sci. Technol. A 14(2), 336, 1996; Poenar D., Optical properties of thin filmsilicon-compatible materials, Appl. Opt. 36 (21), 5112, 1997; HoffmannM., Low-loss fiber-matched low-temperature PECVD waveguides withsmall-core dimensions for optical communication systems, IEEE PhotonicsTech. Lett., 9 (9), 1238, 1997; Pereyra I., High quality low temperatureDPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225, 1997;Kenyon T., A luminescence study of silicon-rich silica and rare-earthdoped silicon-rich silica, Fourth Int. Symp. Quantum ConfinementElectrochemical Society, 97-11, 304, 1997; Alayo M., Thick SiOxNy andSiO2 films obtained by PECVD technique at low temperatures, Thin SolidFilms, 332, 40, 1998; Bulla D., Deposition of thick TEOS PECVD siliconoxide layers for integrated optical waveguide applications, Thin SolidFilms, 334, 60, 1998; Valette S., State of the art of integrated opticstechnology at LETI for achieving passive optical components, J. ofModern Optics, 35 (6), 993, 1988; Ojha S., Simple method of fabricatingpolarization-insensitive and very low crosstalk AWG grating devices,Electron. Lett., 34 (1), 78, 1998; Johnson C., Thermal annealing ofwaveguides formed by ion implantation of silica-on-Si, NuclearInstruments and Methods in Physics Research, B141, 670, 1998; Ridder R.,Silicon oxynitride planar waveguiding structures for application inoptical communication, IEEE J. of Sel. Top. In Quantum Electron., 4 (6),930, 1998; Germann R., Silicon-oxynitride layers for optical waveguideapplications, 195^(th) meeting of the Electrochemical Society, 99-1, May1999, Abstract 137, 1999; Worhoff K., Plasma enhanced cyhemical vapordeposition silicon oxynitride optimized for application in integratedoptics, Sensors and Actuators, 74, 9, 1999; and Offrein B., Wavelengthtunable optical add-after-drop filter with flat passband for WDMnetworks, IEEE Photonics Tech. Lett., 11 (2), 239, 1999

[0008] A comparison of these various PECVD techniques is summarised inFIG. 1 which shows the approaches and methods used to modify the‘delta-n’ between buffer (clad) and core with post-deposition thermaltreatment.

[0009] The various techniques can be grouped into main categories: PECVDusing unknown chemicals, unknown chemical reactions and unknown boron(B) and/or phosphorus (P) chemicals and unknown chemical reactions toadjust the ‘delta-n’ (When specified, the post-deposition thermaltreatments range from 400 to 1000° C.); PECVD using TEOS and unknownmeans of adjusting the ‘delta-n’ (The post-deposition thermal treatmentsare not specified); PECVD using oxidation of SiH₄ with O₂ coupled withsilicon ion implantation or adjustment of silicon oxide stoichiometry asmeans of adjusting the ‘delta-n’ (The post-deposition thermal treatmentsrange from 400 to 1000° C.); PECVD using oxidation of SiH₄ with O₂coupled with the incorporation of CF₄ (SiH₄/O₂/CF₄ flow ratio) as meansof adjusting the ‘delta-n’ (When specified, the post-deposition thermaltreatments range from 100 to 1000° C.); PECVD using oxidation of SiH₄with N₂O coupled with variations of N₂O concentration (SiH₄/N₂O flowratio) as means of adjusting the silicon oxide stoechiometry and the‘delta-n’ (The post-deposition thermal treatments range from 400 to1100° C.); PECVD using oxidation of SiH₄ with N₂O coupled withvariations of N₂O concentration and with the incorporation of Ar(SiH₄/N₂O/Ar flow ratio) as means of adjusting the silicon oxidestoechiometry and the ‘delta-n’ (The post-deposition thermal treatmentsis 1000° C.); PECVD using oxidation of SiH₄ with N₂O coupled with theincorporation of NH₃ (SiH₄/N₂O/NH₃ flow ratio) to form siliconoxynitrides with various ‘delta-n’ (When specified, the post-depositionthermal treatments range from 700 to 1100° C.); PECVD using oxidation ofSiH₄ with N₂O coupled with the incorporation of NH₃ and Ar(SiH₄/N₂O/NH₃/Ar flow ratio) as to form silicon oxynitrides with various‘delta-n’ (The post-deposition thermal treatments are not specified);PECVD using oxidation of SiH₄ with N₂O coupled with the incorporation ofNH₃ and N₂ chemicals variation (SiH₄/N₂O/NH₃/N₂ flow ratio) as to formsilicon oxynitrides with various ‘delta-n’ (The post-deposition thermaltreatments range from 850 to 1150° C.); PECVD using oxidation of SiH₄with N₂O and O₂ coupled with the incorporation of CF₄, N₂ and He(SiH₄/(N₂O/N₂)/ O₂/CF₄ flow ratio) as to form complex mixtures of carbonand fluorine containing silicon oxide as means of adjusting the‘delta-n’ (The post-deposition thermal treatments is 425° C.).

[0010] Our co-pending U.S. patent application Ser. No. 09/833,711entitled ‘Optical Quality Silica Films’ describes an improved PlasmaEnhanced Chemical Vapour Deposition technique for silica films whichshows that the independent control of the SiH₄, N₂O and N₂ gases as wellas of the total deposition pressure via an automatic control of thepumping speed of the vacuum pump in a five-dimensional space consistingof a first independent variable, the SiH₄ flow; a second independentvariable, the N₂O flow; a third independent variable, the N₂ flow; afourth independent variable; the total deposition pressure (controlledby an automatic adjustment of the pumping speed); and the observed filmcharacteristics; permits the elimination of the undesirable residualSi:N—H oscillators (observed as a FTIR peak centered at 3380 cm⁻¹ whose2^(nd) harmonics could cause an optical absorption between 1.445 and1.515 μm), SiN—H oscillators (centered at 3420 cm⁻¹ whose 2^(nd)harmonics could cause an optical absorption between 1.445 and 1.479 μm)and SiO—H oscillators (centered at 3510 cm⁻¹ and whose 2^(nd) harmonicscould cause an optical absorption between 1.408 and 1.441 μm) afterthermal treatment at a low post-deposition temperature of 800° C. toprovide improved silica films with reduced optical absorption in the1.55 μm wavelength (and/or 1.30 μm wavelength) optical region.

[0011] Another co-pending U.S. patent application Ser. No. 09/867,662entitled ‘Method of Depositing Optical Films” describes a new improvedPlasma Enhanced Chemical Vapour Deposition technique of silicawaveguides which shows that the independent control of the SiH₄, N₂O, N₂and PH₃ gases as well as of the total deposition pressure via anautomatic control of the pumping speed of the vacuum pump in asix-dimensional space, namely a first independent variable, the SiH₄flow; a second independent variable, the N₂O flow; a third independentvariable, the N₂ flow; a fourth independent variable, the PH₃ flow; afifth independent variable; the total deposition pressure (controlled byan automatic adjustment of the pumping speed); and the observedwaveguides characteristics, is key to achieving the required ‘delta-n’while still eliminating the undesirable residual Si:N—H oscillators(observed as a FTIR peak centered at 3380 cm¹ whose 2^(nd) harmonicscould cause an optical absorption between 1.445 and 1.515 μm), SiN—Hoscillators (centered at 3420 cm⁻¹ whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.479 μm) and SiO—H oscillators(centered at 3510 cm⁻¹ and whose 2^(nd) harmonics could cause an opticalabsorption between 1.408 and 1.441 μm) after thermal treatment at a lowpost-deposition temperature of 800° C. as to provide improved silicawaveguides with reduced optical absorption in the 1.55 μm wavelength(and/or 1.30 wavelength) optical region.

[0012] While these techniques are capable of producing optical qualityfilms, they can result in stress-induced mechanical problems fordeep-etched optical components.

SUMMARY OF THE INVENTION

[0013] According to the present invention there is provided a method ofdepositing an optical quality silica film by PECVD (Plasma EnhancedChemical Vapor Deposition), comprising independently setting apredetermined flow rate for a raw material gas; independently setting apredetermined flow rate for an oxidation gas; independently setting apredetermined flow rate for a carrier gas; independently setting apredetermined total deposition pressure; and applying a post depositionheat treatment to the deposited film at a temperature selected tooptimize the mechanical properties without affecting the opticalproperties of the deposited film.

[0014] In a preferred embodiment flow rate for a dopant gas is alsoindependently set. The observed FTIR characteristics of the depositedfilm are monitored to determine the optimum post deposition heattreatment temperature.

[0015] This technique permits the required ‘delta-n’ to be achievedwhile eliminating the undesirable residual Si:N—H oscillators (observedas a FTIR peak centered at 3380 cm⁻¹ whose 2^(nd) harmonics could causean optical absorption between 1.445 and 1.515 μm), SiN—H oscillators(centered at 3420 cm⁻¹ whose 2^(nd) harmonics could cause an opticalabsorption between 1.445 and 1.479 μm) and SiO—H oscillators (centeredat 3510 cm⁻¹ and whose 2^(nd) harmonics could cause an opticalabsorption between 1.408 and 1.441 μm) after an optimised thermaltreatment in a nitrogen. The technique can provide improved silica-basedoptical elements with reduced optical absorption in the 1.55 μmwavelength (and/or 1.30 μm wavelength) optical region without theresidual stress-induced mechanical problems of deep-etched opticalelements (mechanical movement of sidewalls), without the residualstress-induced mechanical problems at the buffer/core or core/claddinginterfaces (micro-structural defects, micro-voiding and separation) andwithout the residual stress-induced optical problems (polarisationdependant power loss).

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will now be described in more detail, by way ofexample only, with reference to the accompanying drawings, in which:

[0017]FIG. 1 is a comparison table showing various PECVD approaches forcontrolling the refractive index and reducing the optical absorption ofsilica films;

[0018]FIG. 2 shows the FTIR fundamental infrared absorption peaks andtheir corresponding higher harmonics peaks associated with the residualcompounds resulting from high temperature thermal treatments of PECVDsilica-based optical components in a nitrogen ambient;

[0019]FIG. 3a shows the basic FTIR spectra of various buffers(claddings) obtained with a typical PECVD process after a 180 minutesthermal treatment in a nitrogen ambient at various temperatures;

[0020]FIG. 3b shows the basic FTIR spectra of various buffers(claddings) obtained with the PECVD deposition technique described inour co-pending U.S. patent application Ser. No. 09/833,711 and after athermal treatment in a nitrogen ambient at 800° C.;

[0021]FIG. 3c shows the basic FTIR spectra of various cores obtained at2.60 Torr with the PECVD deposition technique described in ourco-pending U.S. patent application Ser. No. 09/799,4091 and after athermal treatment in a nitrogen ambient at 800° C.;

[0022]FIG. 3d shows the basic FTIR spectra of various cores obtainedwith the PECVD deposition technique in accordance with the principles ofthe invention and after a 30 minutes thermal treatment in a nitrogenambient at various temperatures;

[0023]FIG. 4a shows the in-depth FTIR spectra from 810 to 1000 cm⁻¹ ofvarious buffers (clads) obtained with a typical PECVD process after a180 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0024]FIG. 4b shows the in-depth FTIR spectra from 810 to 1000 cm⁻¹ ofvarious buffers (clads) obtained with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/833,711titled after a thermal treatment in a nitrogen ambient at 800° C.;

[0025]FIG. 4c shows the in-depth FTIR spectra from 810 to 1000 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C.;

[0026]FIG. 4d shows the in-depth FTIR spectra from 810 to 1000 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0027]FIG. 5c shows the in-depth FTIR spectra from 1260 to 1500 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C.;

[0028]FIG. 5d shows the in-depth FTIR spectra from 1260 to 1500 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0029]FIG. 6a shows the in-depth FTIR spectra from 1500 to 1600 cm⁻¹ ofvarious buffers (claddings) obtained with a typical PECVD process aftera 180 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0030]FIG. 6b shows the in-depth FTIR spectra from 1500 to 1600 cm⁻¹ ofvarious buffers (claddings) obtained with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/833,711after a thermal treatment in a nitrogen ambient at 800° C.;

[0031]FIG. 6c shows the in-depth FTIR spectra from 1500 to 1600 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C.;

[0032]FIG. 6d shows the in-depth FTIR spectra from 1500 to 1600 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0033]FIG. 7a shows the in-depth FTIR spectra from 1700 to 2200 cm⁻¹ ofvarious buffers (claddings) obtained with a typical PECVD process aftera 180 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0034]FIG. 7b shows the in-depth FTIR spectra from 1700 to 2200 cm⁻¹ ofvarious buffers (claddings) obtained with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/833,711after a thermal treatment in a nitrogen ambient at 800° C.;

[0035]FIG. 7c shows the in-depth FTIR spectra from 1700 to 2200 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C;

[0036]FIG. 7d shows the in-depth FTIR spectra from 1700 to 2200 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0037]FIG. 8a shows the in-depth FTIR spectra from 2200 to 2400 cm⁻¹ ofvarious buffers (cladding) obtained with a typical PECVD process after a180 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0038]FIG. 8b shows the in-depth FTIR spectra from 2200 to 2400 cm⁻¹ ofvarious buffers (claddings) obtained with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/833,711after a thermal treatment in a nitrogen ambient at 800° C.;

[0039]FIG. 8c shows the in-depth FTIR spectra from 2200 to 2400 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C.;

[0040]FIG. 8d shows the in-depth FTIR spectra from 2200 to 2400 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0041]FIG. 9a shows the in-depth FTIR spectra from 3200 to 3900 cm⁻¹ ofvarious buffers (claddings) obtained with a typical PECVD process aftera 180 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0042]FIG. 9b shows the in-depth FTIR spectra from 3200 to 3900 cm⁻¹ ofvarious buffers (claddings) obtained with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/833,711after a thermal treatment in a nitrogen ambient at 800° C.;

[0043]FIG. 9c shows the in-depth FTIR spectra from 3200 to 3900 cm⁻¹ ofvarious cores obtained at 2.60 Torr with the PECVD deposition techniquedescribed in our co-pending U.S. patent application Ser. No. 09/799,491after a thermal treatment in a nitrogen ambient at 800° C.;

[0044]FIG. 9d shows the in-depth FTIR spectra from 3200 to 3900 cm⁻¹ ofvarious cores obtained with the new PECVD deposition technique after a30 minutes thermal treatment in a nitrogen ambient at varioustemperatures;

[0045]FIG. 10 shows the stress hysteresis of buffer (cladding) and corein a nitrogen ambient using a 180 minutes stabilization at 800° C.;

[0046]FIG. 11 is SEM pictures of a grating and of a waveguide withquasi-vertical side-walls deep-etched through buffer and core;

[0047]FIG. 12 shows the gradually sloped side-wall formation from theelastic strain of deep-etched buffer/core optical elements resultingfrom the (compressive stress buffer)/(tensile stress core) combination;

[0048]FIG. 13 shows side-wall angle measurements of neighboring 5.0 μmwide deep-etched waveguide and a 1150 μm wide deep-etched grating; FIGS.13a and 13 b show the relative position between an isolated 5.0 μm widedeep-etched waveguide and its neighboring 1150 μm wide deep-etchedgrating at two different magnifications; FIG. 13c shows the sidewall ofthe 5.0 μm wide deep-etched waveguide facing the neighboring grating hasa slope of about 90°; FIG. 13d shows the side-wall of the 1150 μm widedeep-etched grating facing the neighboring deep-etched waveguide has amuch smaller slope of about 84°;

[0049]FIG. 14 shows how the interfacial stress relief of the shearstress building at the buffer/core or core/clading interfaces results ina noticeable modification of the microstructure of these interfaces;

[0050]FIG. 15 shows how the interfacial stress relief of the shearstress building at the buffer/core or core/cladding interfaces resultsin an important modification of the micro-structure and in the formationof micro-voids in the core and near these interfaces;

[0051]FIG. 16 shows the stress relief contraction of the tensile stresscore during SEM preparation;

[0052]FIG. 17 shows the effect of the incidence angle of infrared lightat the air/core interface on the reflection and transmission of infraredoptical power (case where the infrared light is incoming from the airside of the side-wall of core a waveguide, a grating or of an anotheroptical element); and

[0053]FIG. 18 shows the effect of the incidence angle of infrared lightat the air/core interface on the reflection and transmission of infraredoptical power (case where the infrared light is incoming from the coreside of the side-wall of core a waveguide, a grating or of an anotheroptical element).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] The invention can be implemented to create PECVD optical qualitysilica-based optical elements using a commercially available PECVDsystem, the “Concept One” system manufactured by Novellus Systems inCalifornia, USA, and a standard diffusion tube.

[0055]FIG. 2 lists some FTIR fundamental infrared absorption peaks andtheir corresponding higher harmonics peaks associated with the variousresidual compounds resulting the Plasma Enhanced Chemical VapourDeposition (PECVD) of buffer (cladding) from a silane (SiH₄) and nitrousoxide (N₂O) gas mixture at a relatively low temperature of 400° C. usingthe following reaction:

SiH₄(g)+2N₂O(g)→SiO₂+2N₂(g)+2H₂(g)

[0056] and following high temperature thermal treatments in a nitrogenambient. It will be seen that the FTIR fundamental infrared absorptionpeaks and their corresponding higher harmonics peaks associated of theresidual compounds resulting from high temperature thermal treatments ofPECVD silica films in a nitrogen ambient will contribute to the opticalabsorption in the 1.30 to 1.55 μm optical bands. The second vibrationharmonics of the HO—H oscillators in trapped water vapour in themicro-pores of the silica films (3550 to 3750 cm⁻¹) increase the opticalabsorption near 1.333 to 1.408 μm. The second vibration harmonics of theSiO—H oscillators in the silica films (3470 to 3550 cm⁻¹) increases theoptical absorption near 1.408 to 1.441 μm. The second vibrationharmonics of the Si:N—H oscillators in the silica films (3300 to 3460cm⁻¹) increases the optical absorption near 1.445 to 1.515 μm. Thesecond vibration harmonics of the SiN—H oscillators in the silica films(3380 to 3460 cm⁻¹) increases the optical absorption near 1.445 to 1.479μm. The third vibration harmonics of the Si—H oscillators in the silicafilms (2210 to 2310 cm⁻¹) increases the optical absorption near 1.443 to1.505 μm. The fourth vibration harmonics of the Si═O oscillators in thesilica films (1800 to 1950 cm⁻¹) increases the optical absorption near1.282 to 1.389 μm. The fifth vibration harmonics of the N═N oscillatorsin the silica films (1530 to 1580 cm⁻¹) increases the optical absorptionnear 1.266 to 1.307 μm.

[0057] The negative effects of these the oscillators on the opticalproperties of silica-based optical components are reported in theliterature. See, for example, Grand G., Low-loss PECVD silica channelwaveguides for optical communications, Electron. Lett., 26 (25), 2135,1990; Bruno F., Plasma-enhanced chemical vapor deposition of low-lossSiON optical waveguides at 1.5-μm wavelength, Applied Optics, 30 (31),4560, 1991; Imoto K., High refractive index difference and low lossoptical waveguide fabricated by low temperature processes, ElectronicLetters, 29 (12), 1993; Hoffmann M., Low temperature, nitrogen dopedwaveguides on silicon with small core dimensions fabricated byPECVD/RIE, ECIO'95, 299, 1995; Bazylenko M., Pure and fluorine-dopedsilica films deposited in a hollow cathode reactor for integrated opticapplications, J. Vac. Sci. Technol. A 14 (2), 336, 1996; Pereyra I.,High quality low temperature DPECVD silicon dioxide, J. Non-CrystallineSolids, 212, 225, 1997; Kenyon T., A luminescence study of silicon-richsilica and rare-earth doped silicon-rich silica, Electrochem. Soc. Proc.Vol. 97-11, 304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained byPECVD technique at low temperatures, Thin Solid Films, 332, 40, 1998.Germann R., Silicon-oxynitride layers for optical waveguideapplications, 195^(th) meeting of the Electrochemical Society, 99-1, May1999, Abstract 137, 1999; Worhoff K., Plasma enhanced chemical vapordeposition silicon oxynitride optimized for application in integratedoptics, Sensors and Actuators, 74, 9, 1999.

[0058] This literature describes the tentative elimination of opticalabsorption (i.e. of the six residual oscillators) using thermaldecomposition reactions during thermal treatments under a nitrogenambient at a maximum temperature lower than 1350° C., the fusion pointof the silicon wafer.

COMPARATIVE EXAMPLES

[0059] Optical absorption of typical PECVD buffer (cladding) following a180 minutes thermal treatment in a nitrogen ambient at various hightemperatures

[0060]FIG. 3a, FIG. 4a, FIG. 6a, FIG. 7a, FIG. 8a and FIG. 9a show theFTIR spectra of typically deposited PECVD silica films before and aftera 180 minutes long high temperature thermal treatment in a nitrogenambient at a temperature of either 600, 700, 800, 900, 1000 or 1100° C.It can be seen that the higher the thermal decomposition temperature ofthe high temperature thermal treatment in a nitrogen ambient, the betterthe basic FTIR spectra of the treated silica films.

[0061]FIG. 3a shows the expected gradually more intense and smaller FWHMSi—O—Si “rocking mode” absorption peak (centred at 460 cm⁻¹) and Si—O—Si“in-phase-stretching mode” absorption peak (centred at 1080 cm⁻¹) as thetemperature of the 180 minutes long thermal treatment in a nitrogenambient is increased from 600° C. to 1100° C.

[0062]FIG. 4a shows that the elimination of the Si—OH oscillators(centered at 885 cm⁻¹) is easy and already complete after the 180minutes long thermal treatment in a nitrogen ambient at 600° C. FIG. 4aalso shows that the elimination of the Si—ON oscillators (centred at 950cm⁻¹) is much more difficult and that the higher the temperature of the180 minutes long thermal treatment in a nitrogen ambient, the morenitrogen incorporation as Si—ON oscillators (i.e. as SiONH and/or SiON₂compounds).

[0063]FIG. 6a shows that the elimination of the N═N oscillators(centered at 1555 cm⁻¹) is also very difficult and does require thetemperature of the high temperature thermal treatment in a nitrogenambient to reach 1000° C.

[0064]FIG. 7a shows that there is very little influence of thetemperature of the high temperature thermal treatment in a nitrogenambient on the Si═O oscillators (centered at 1875 cm⁻¹) and on theunknown oscillator (centered at 2010 cm⁻¹).

[0065]FIG. 8a shows that the elimination of the Si-H oscillators(centered at 2260 cm⁻¹ and whose 3^(rd) harmonics could cause an opticalabsorption between 1.443 and 1.508 μm) is easy and already completeafter the 180 minutes long thermal treatment in a nitrogen ambient at600° C.

[0066]FIG. 9a shows that the elimination of the Si:N—H oscillators(centered at 3380 cm⁻¹ whose 2^(nd) harmonics could cause an opticalabsorption between 1.445 and 1.515 μm) is also very difficult and doesrequire the temperature of the high temperature thermal treatment in anitrogen ambient to reach 1100° C. The complete elimination of theSi:N—H oscillators is extremely difficult because the nitrogen atoms ofthese oscillators are bonded to the silicon atoms of the SiO₂ networkvia two covalent bonds. FIG. 9a also shows that the elimination of theSiN—H oscillators (centered at 3420 cm⁻¹ whose 2^(nd) harmonics couldcause an optical absorption between. 1.445 and 1.479 μm) is almost asdifficult and does require the temperature of the high temperaturethermal treatment in a nitrogen ambient to reach 1000° C. FIG. 9a alsoshows that the elimination of the SiO—H oscillators (centered at 3510cm⁻¹ and whose 2^(nd) harmonics could cause an optical absorptionbetween 1.408 and 1.441 μm) is slightly easier and does require thetemperature of the high temperature thermal treatment in a nitrogenambient to reach 900° C. Finally, FIG. 9a also shows that theelimination of the HO—H oscillators (centered at 3650 cm⁻¹ and whose2^(nd) harmonics could cause an optical absorption between 1.333 and1.408 μm) is very easy since already complete after the high temperaturethermal treatment in a nitrogen ambient of only 600° C.

[0067] It is apparent from the various FTIR spectra that it is necessaryto use extremely high temperature thermal treatments in a nitrogenambient in order to eliminate the residual optical absorption oftypically deposited PECVD silica films. In particular, it isdemonstrated that the elimination of the residual nitrogen and hydrogenof typically deposited PECVD silica films is very difficult since theresidual Si:N—H oscillators (whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.515 μm) requires a temperature of1100° C. because the nitrogen atoms of these oscillators are bonded tothe silicon atoms of the SiO₂ network via two covalent bonds, theelimination of the SiN—H oscillators (whose 2^(nd) harmonics could causean optical absorption between 1.445 and 1.479 μm) requires a temperatureof 1000° C., and the elimination of the SiO—H oscillators (whose 2^(nd)harmonics could cause an optical absorption between 1.408 and 1.441 μm)requires a temperature of 900° C.

[0068] It is very difficult to achieve high optical quality silica-basedoptical components from typically deposited PECVD silica films usingthermal treatments in nitrogen ambient at temperatures lower than 1100°C.

[0069] Our co-pending U.S. patent application Ser. No. 09/833,711describes an improved Plasma Enhanced Chemical Vapour Depositiontechnique for silica films which involves the independent control of theSiH₄, N₂O and N₂ gases as well as of the total deposition pressure viaan automatic control of the pumping speed of the vacuum pump in afive-dimensional space. The first independent variable, the SiH₄ gasflow, is fixed at 0.20 std litre/min. The second independent variable,the N₂O gas flow, is fixed at 6.00 std litre/min. The third independentvariable, the N₂ gas flow, being fixed at 3.15 std litre/min. The fourthindependent variable, the total deposition pressure, being variedbetween of 2.00 Torr, 2.10 Torr, 2.20 Torr, 2.30 Torr, 2.40 Torr, 2.50Torr, and 2.60 Torr. The fifth dimension is the observed FTIRcharacteristics of various buffers (claddings), as reported in FIG. 3b,FIG. 4b, FIG. 6b, FIG. 7b, FIG. 8b and FIG. 9b.

[0070] The five-dimensional space permits the elimination of theseresidual nitrogen and hydrogen atoms as to achieve high optical qualitysilica-based optical components from typically deposited PECVD silicafilms a 180 minutes thermal treatment in a nitrogen ambient at a reducedtemperature of 800° C.

[0071]FIG. 3b, FIG. 4b, FIG. 6b, FIG. 7b, FIG. 8b and FIG. 9b show theFTIR spectra of PECVD silica films deposited using a commerciallyavailable PECVD system, the “Concept One” system manufactured byNovellus Systems in California, USA, using the fixed flow rates ofsilane (SiH₄), of nitrous oxide (N₂O) and of nitrogen (N₂O), asdescribed in this co-pending U.S. patent application Ser. No.09/833,711. These spectra are obtained before and after a 180 minutesthermal treatment in a nitrogen ambient at a reduced temperature of 800°C. in a standard diffusion tube. It is clear that the techniquedescribed in our co-pending application allows the attainment of highoptical quality silica films after a 180 minutes thermal treatment in anitrogen ambient at a reduced temperature of 800° C. and that theindependent control of the downstream pressure of this improved PECVDdeposition technique has a major effect on the FTIR spectra of thetreated silica films:

[0072]FIG. 3b shows a more intense and smaller FWHM Si—O—Si “rockingmode” absorption peak (centred at 460 cm⁻¹) and Si—O—Si“in-phase-stretching mode” absorption peak (centred at 1080 cm⁻¹) as thetotal deposition pressure is increased from 2.00 Torr to 2.40 Torrfollowed by a slight degradation as the pressure is increased furthermore up to 2.60 Torr;

[0073]FIG. 4b shows the gradual elimination of the Si—OH oscillators(centered at 885 cm⁻¹) as the total deposition pressure is increasedfrom 2.00 Torr up to the optimum pressure of 2.40 Torr followed by aslight degradation as the pressure is increased further more up to 2.60Torr. FIG. 4b also shows the gradual elimination of the Si—ONoscillators (centred at 950 cm⁻¹) as the total deposition pressure isincreased from 2.00 Torr to 2.40 Torr followed by a slight degradationas the pressure is increased further more up to 2.60 Torr. The optimumseparation and deep valley observed at 2.40 Torr is an indication thatthe silica films resulting from this optimum deposition pressure arecomposed of high quality SiO₂ material. This contrasts with theupper-mentioned results of typical PECVD silica films which stillincorporate a lot of Si—ON oscillators even after much highertemperature thermal treatments in a nitrogen ambient;

[0074]FIG. 6b shows the gradual and total elimination of the N═Noscillators (centered at 1555 cm⁻¹) as the total deposition pressure isincreased from 2.00 Torr to 2.60 Torr. This also contrasts with theupper-mentioned results of typical PECVD silica films which require a180 minutes thermal treatment in a nitrogen ambient at a temperature of1000° C. in order to achieve similar results;

[0075]FIG. 7b shows the gradual elimination of the Si═O oscillators(centered at 1875 cm⁻¹) and on the unknown oscillator (centered at 2010cm⁻¹) as the total deposition pressure is increased from 2.00 Torr to2.40 Torr followed by a slight degradation as the pressure is increasedfurther more up to 2.60 Torr. These effects are not that important sinceonly the fourth harmonics of the Si═O oscillators could absorb in the1.30 to 1.55 μm optical bands;

[0076]FIG. 8b shows that the Si—H oscillators (centered at 2260 cm⁻¹which 3^(rd) harmonics could cause an optical absorption between 1.443and 1.508 μm) are completely eliminated for all deposition pressures;

[0077]FIG. 9b shows the spectacular gradual elimination of the Si:N—Hoscillators (centered at 3380 cm⁻¹ whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.515 μm) as the total depositionpressure is increased from 2.00 Torr to 2.60 Torr. This contrasts withthe upper-mentioned results of typical PECVD silica films which requirea thermal treatment in a nitrogen ambient at a temperature of 1100° C.in order to achieve similar results. FIG. 9b also shows a spectaculargradual elimination of the SiN—H oscillators (centered at 3420 cm⁻¹whose 2^(nd) harmonics could cause an optical absorption between 1.445and 1.479 μm) as the total deposition pressure is increased from 2.00Torr to 2.60 Torr. This also contrasts with the upper-mentioned resultsof typical PECVD silica films which require a thermal treatment in anitrogen ambient at a temperature of 1000° C. in order to achievesimilar results. FIG. 9b also shows that the SiO—H oscillators (centeredat 3510 cm⁻¹ and whose 2^(nd) harmonics could cause an opticalabsorption between 1.408 and 1.441 μm) are completely eliminated for alldeposition pressures. This also contrasts with the upper-mentionedresults of typical PECVD silica films which require a thermal treatmentin a nitrogen ambient at a temperature of 900° C. in order to achievesimilar results. Finally, FIG. 9b also shows that the elimination of theHO—H oscillators (centered at 3650 cm⁻¹ and whose 2^(nd) harmonics couldcause an optical absorption between 1.333 and 1.408 μm) are completelyeliminated for all deposition pressures.

[0078] It is apparent from the various FTIR spectra that our co-pendingU.S. patent application Ser. No. 09/833,711 prohibits the use ofextremely high temperature thermal treatments in a nitrogen ambient inorder to eliminate the residual optical absorption of typicallydeposited PECVD silica films. In particular, it is demonstrated that theelimination of the residual nitrogen and hydrogen of typically depositedPECVD silica films is completely achieved after a 180 minutes thermaltreatment in a nitrogen ambient at a reduced temperature of 800° C. Theresidual Si:N—H oscillators (whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.515 μm) are completely eliminatedas the total deposition pressure is increased from 2.00 Torr to 2.60Torr. The residual SiN—H oscillators (whose 2^(nd) harmonics could causean optical absorption between 1.445 and 1.479 μm) are also completelyeliminated as the total deposition pressure is increased from 2.00 Torrto 2.60 Torr. The residual SiO—H oscillators (whose 2^(nd) harmonicscould cause an optical absorption between 1.408 and 1.441 μm) are alsocompletely eliminated as the total deposition pressure is increased from2.00 Torr to 2.60 Torr.

[0079] It is then very easy to achieve high optical quality silica filmsafter a 180 minutes thermal treatment in a nitrogen ambient at a reducedtemperature of 800° C. using the technique described in our co-pendingU.S. patent application Ser. No. 09/833,711.

[0080] Our co-pending U.S. patent application Ser. No. 09/799,491 showsthe spectacular effect of a fifth independent variable, the phosphine,PH₃ gas flow, on the optimization of the optical properties of thevarious buffer (cladding) and core waveguides in a six-dimensionalspace. The first independent variable, the SiH₄ gas flow, is fixed at0.20 std litre/mi. The second independent variable, the N₂O gas flow, isfixed at 6.00 std litre/min. The third independent variable, the N₂ gasflow, is fixed at 3.15 std litre/min. The fourth independent variable,the PH₃ gas flow, is varied between 0.00 std litre/min, 0.12 stdlitre/min; 0.25 std litre/min; 0.35 std litre/min; 0.50 std litre/min;and 0.65 std litre/min.

[0081] The fifth independent variable, the total deposition pressure, isfixed at 2.60 Torr

[0082] The sixth dimension is the observed FTIR characteristics ofvarious buffer (cladding) and core waveguides, as reported in: FIG. 3c,FIG. 4c, FIG. 5c, FIG. 6c, FIG. 7c, FIG. 8c, & FIG. 9c.

[0083]FIG. 3c, FIG. 4c, FIG. 5c, FIG. 6c, FIG. 7c, FIG. 8c and FIG. 9cshow the FTIR spectra of PECVD silica films deposited using acommercially available PECVD system, the “Concept One” systemmanufactured by Novellus Systems in California, USA, using the fixedoptimum total deposition pressure and the fixed flow rates of silane(SiH₄), of nitrous oxide (N₂O) and of nitrogen (N₂O), as described inour co-pending U.S. patent application Ser. No. 09/799,491. Thesespectra are obtained after a high temperature thermal treatment for 180minutes in a nitrogen ambient at a fixed temperature of only 800° C in astandard diffusion tube. It is clear that the technique described in ourco-pending patent application allows the achievement of high opticalquality silica waveguides after a 180 minutes thermal treatment in anitrogen ambient at a reduced temperature of 800° C.:

[0084]FIG. 3c shows that the intense and small FWHM Si—O—Si “rockingmode” absorption peak (centred at 460 cm⁻¹) and Si—O—Si“in-phase-stretching mode” absorption peak (centred at 1080 cm⁻¹) of thefixed deposition pressure of 2.60 Torr of FIG. 3b is maintained in FIG.3c as the PH₃ flow rate is gradually increased from 0.00 std litre/minto 0.65 std litre/min. This means that at a fixed deposition pressure of2.60 Torr, the control of the PH₃ gas flow independently of the SiH₄ gasflow, of the N₂O gas flow and of the N₂ gas flow has no effect on thebasic FTIR spectra of the treated silica films;

[0085]FIG. 4c shows that an even more gradual elimination of the Si—OHoscillators (centered at 885 cm⁻¹) is observed at the total depositionpressure of 2.60 Torr as the PH₃ flow rate is increased from 0.00 stdlitre/min to 0.65 std litre/min. FIG. 4c also shows that a gradualelimination of the Si—ON oscillators (centred at 950 cm⁻¹) is alsoobserved at the total deposition pressure of 2.60 Torr as the PH₃ flowrate is increased from 0.00 std litre/min up to the optimum 0.25 stdlitre/min followed by a very slight degradation as the PH₃ flow rate isincreased further more up to 0.65 std litre/min. This spectacularimproved elimination of the residual Si—ON oscillators after a 180minutes thermal treatment of only 800° C. contrasts with theupper-mentioned results of typical PECVD silica films of FIG. 4a whichstill incorporate a lot of Si—ON oscillators even after a thermaltreatment in a nitrogen ambient at a much higher temperature of 1100° C.This also contrasts with the upper-mentioned results of PECVD buffer(cladding) deposited at a non-optimized pressure of less than 2.40 Torras described in our co-pending U.S. patent application Ser. No.09/833,711 of FIG. 4b which still incorporate a large number of Si—ONoscillators even after a 180 minutes thermal treatment in a nitrogenambient at a much higher temperature of 800° C. The optimum separationand deep valley between the Si—O—Si “in-phase-stretching mode”absorption peak (1080 cm⁻¹) and the Si—O—Si “bending mode” absorptionpeak (810 cm⁻¹) of the fixed deposition pressure of 2.60 Torr of FIG. 4bis maintained and in fact slightly improved as the PH₃ flow rate isgradually increased from 0.00 std litre/min to 0.35 std litre/min.

[0086]FIG. 5c shows that a gradual appearance of the P═O oscillators(centered at 1330 cm⁻¹ and which does not have a higher harmonics whichcould cause optical absorption in the 1.30 to 1.55 μm optical bands) isobserved at the total deposition pressure of 2.60 Torr as the PH₃ flowrate is increased from 0.00 std litre/min to 0.65 std litre/min. ThisFTIR absorption peak is used to calibrate the phosphorus incorporationin core.

[0087]FIG. 6c shows that of the N═N oscillators (centered at 1555 cm⁻¹)are completely eliminated at the total deposition pressure of 2.60 Torrfor all PH₃ flow rate values from 0.00 std litre/min to 0.65 stdlitre/min. This contrasts with the upper-mentioned results of typicalPECVD silica films of FIG. 6a which require a 180 minutes thermaltreatment in a nitrogen ambient at a temperature of 1000° C. in order toachieve similar results. This also contrasts with the upper-mentionedresults of PECVD buffer (cladding) deposited at a non-optimized pressureof less than 2.40 Torr by our co-pending U.S. patent application Ser.No. 09/833,711' of FIG. 6b which still incorporate a large number of N═Noscillators even after a 180 minutes thermal treatment in a nitrogenambient at a much higher temperature of 800° C.

[0088]FIG. 7c shows that the Si═O oscillators (centered at 1875 cm⁻¹)and the unknown oscillator (centered at 2010 cm⁻¹) at the totaldeposition pressure of 2.60 Torr are not influenced by the PH₃ flow ratefrom 0.00 std litre/min to 0.65 std litre/min. These effects are notthat important since only the fourth harmonics of the Si═O oscillatorscould absorb in the 1.30 to 1.55 μm optical bands;

[0089]FIG. 8c shows that the Si—H oscillators (centered at 2260 cm⁻¹ andwhich third harmonics could cause an optical absorption between 1.443and 1.508 μm) at the total deposition pressure of 2.60 Torr are stillcompletely eliminated by any of all PH₃ flow rates from 0.00 stdlitre/min to 0.65 std litre/min.

[0090]FIG. 9c shows that the complete elimination of the Si:N—Hoscillators (centered at 3380 cm⁻¹ whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.515 μm) at the total depositionpressure of 2.60 Torr is maintained for all PH₃ flow rates from 0.00 stdlitre/min to 0.65 std litre/min. This contrasts with the upper-mentionedresults of typical PECVD silica films which require a thermal treatmentin a nitrogen ambient at a temperature of 1100° C. in order to achievesimilar results. This also contrasts with the upper-mentioned results ofPECVD buffer (cladding) deposited at a non-optimized pressure of lessthan 2.40 Torr by our co-pending U.S. patent application Ser. No.09/833,711 of FIG. 9b which still incorporate a lot of Si:N—Hoscillators even after a 180 minutes thermal treatment in a nitrogenambient at a much higher temperature of 800° C.

[0091]FIG. 9c also shows that the a spectacular complete elimination ofthe SiN—H oscillators (centered at 3420 cm⁻¹ whose 2^(nd) harmonicscould cause an optical absorption between 1.445 and 1.479 μm) at thetotal deposition pressure of 2.60 Torr is also maintained for all PH₃flow rates from 0.00 std litre/min to 0.65 std litre/min. This contrastswith the upper-mentioned results of typical PECVD silica films whichrequire a thermal treatment in a nitrogen ambient at a temperature of1000° C. in order to achieve similar results. This also contrasts withthe upper-mentioned results of PECVD buffer (cladding) deposited at anon-optimized pressure of less than 2.40 Torr by our co-pending U.S.patent application Ser. No. 09/833,711 of FIG. 9b which stillincorporate a lot of SiN—H oscillators even after a 180 minutes thermaltreatment in a nitrogen ambient at a much higher temperature of 800° C.

[0092]FIG. 9c also shows that the complete elimination of the SiO—Hoscillators (centered at 3510 cm⁻¹ whose 2^(nd) harmonics could cause anoptical absorption between 1.408 and 1.441 μm) at the total depositionpressure of 2.60 Torr is maintained for all PH₃ flow rates from 0.00 stdlitre/min to 0.65 std litre/min. This contrasts with the upper-mentionedresults of typical PECVD silica films which require a thermal treatmentin a nitrogen ambient at a temperature of 900° C. in order to achievesimilar results. Finally, FIG. 9c also shows that the completeelimination of the HO—H oscillators (centered at 3650 cm⁻¹ whose 2^(nd)harmonics could cause an optical absorption between 1.333 and 1.408 μm)at the total deposition pressure of 2.60 Torr is maintained for all PH₃flow rates from 0.00 std litre/min to 0.65 std litre/min.

[0093] It is clear from the various FTIR spectra that our co-pendingU.S. patent application Ser. No. 09/799,491 allows the use of variousPH₃ flow rates from 0.00 std litre/min to 0.65 std litre/min. to achievethe required ‘delta-n’ after a 180 minutes thermal treatment in anitrogen ambient at a reduced temperature of 800° C. while maintainingexcellent optical quality.

[0094] However, with this 180 minutes thermal treatment in a nitrogenambient at a reduced temperature of 800° C. are associated some residualstress-induced mechanical problems of deep-etched optical elements(mechanical movement of the side-walls), some residual stress-inducedmechanical problems at the buffer/core interface or at the core/claddinginterface (micro-structural defects, micro-voiding and separation) andsome residual stress-induced optical problems (polarisation dependantpower loss).

[0095]FIG. 10 shows the stress hysteresis in a nitrogen ambient ofbuffer (cladding) and core during the heating of the silicon wafer fromroom temperature to 800° C., during its stabilization for 180 minutes at800° C. and during its natural cooling from 800° C. to room temperature.

[0096]FIG. 10 shows that the mechanical stress of buffer (cladding) iscompressive at about −250 MPa prior to the stress hysteresis cycle; iscompressive throughout the complete stress hysteresis cycle; decreasesalmost linearly as the temperature increases linearly; an expectedsituation since the (almost constant) coefficient of linear expansion ofsilica-based buffer (cladding) is smaller than the one of the underlyingsilicon; and shows three plastic deformation regions during the stresshysteresis cycle, namely Region B1, from 450° C. to 575° C., where itdecreases much faster than what is expected from a linear decreaseassociated with its elastic deformation; Region B2, from 575° C. to 650°C., where it is almost constant; and Region B3, during the 180 minutesstabilization at 800° C., where it decreases as the temperature remainsunchanged. The mechanical stress of buffer (cladding) is alsocompressive at about −150 MPa after the stress hysteresis cycle.

[0097] In addition FIG. 10 shows that the mechanical stress of core istensile at about 175 MPa prior to the stress hysteresis cycle; istensile throughout the complete stress hysteresis cycle; and increasesalmost linearly as the temperature increases linearly. This is anexpected situation since the (almost constant) coefficient of linearexpansion of silica-based core is smaller than the one of the underlyingsilicon.

[0098]FIG. 10 also shows two plastic deformation regions during thestress hysteresis cycle, namely Region C1, from 450° C. to 675° C.,where the stress reverses its trends and in fact decreases as thetemperature is increasing; and Region C2, from 675° C. to 800° C., whereit is almost constant. The stress is tensile at about 40 MPa after thestress hysteresis cycle.

[0099]FIG. 10 shows that the optical elements of the device are to beprepared from a (compressive stress buffer)/(tensile stress core)combination bi-layer after a thermal treatment for 180 minutes in anitrogen ambient at a reduced temperature of 800° C. To this particularcombination are associated some residual stress-induced mechanicalproblems of deep-etched optical elements (mechanical movement ofside-walls), some residual stress-induced mechanical problems at thebuffer/core or core/cladding interfaces (micro-structural defects,micro-voiding and separation) and some residual stress-induced opticalproblems (polarisation dependant power loss).

[0100] Optical elements, such as gratings or waveguides, requiredeep-etched (compressive stress buffer)/(tensile stress core) withvertical side-walls and with a seamless buffer/core interface.

[0101]FIG. 11 shows SEM pictures of a grating and a waveguide withdeep-etched vertical side-walls and with a seamless buffer/coreinterface deep-etched through buffer and core.

[0102]FIG. 12 shows a stress-relief mechanism involving the elasticstrain of such a deep-etched (compressive stress buffer)/(tensile stresscore) optical element. From this sequence of three graphicalrepresentations, it is clear that such a (compressive stressbuffer)/(tensile stress core) deep-etched optical element willsystematically result in a positively sloped elastic strain of theoptical element's side-wall.

[0103] This stress-relieve mechanism shows that the lateral strain ofthe compressive stress buffer forces the deep-etched side-wall of bufferto move outward; and the lateral strain of the tensile stress coreforces the deep-etched side-wall of core to move inward.

[0104] This combination of strains will systematically result indeep-etched (compressive stress buffer)/(tensile stress core) opticalelements with a positive slope side-wall, i.e. a side-wall with an anglesmaller than 90°.

[0105] To estimate the amplitude of this effect, consider thehypothetical of zero bonding at the buffer/(silicon wafer) interface, ofzero bonding at the buffer/core interface, and of zero bonding at thebuffer/core interface. The outward elastic strain of the side-wall ofthe compressive stress buffer, ε_(B), and the inward elastic strain ofthe side-wall of the tensile stress core, ε_(C), would simply be:

ε_(B)=σ_(B) /E _(B); ε_(C)=σ_(C) /E _(C)

[0106] where σ_(B) and E_(B) are respectively the mechanical stress andthe modulus of elasticity of buffer and where σ_(C) and E_(C) arerespectively the mechanical stress and the modulus of elasticity ofcore.

[0107] The modulus of elasticity of silica thin films measured bymicro-indentation and measured by electrostatic membrane deflection arerespectively reported as 70 GPa and 69 GPa in the following tworeferences: Thin Solid Films, Vol. 283, p. 15, (1996); IEEE Transactionson Electron Devices, Vol. ED25, No.10, p.1249, (1978).

[0108] To the −150 MPa compressive stress of buffer and 40 MPa tensilestress of core reported in FIG. 10 at room temperature would then beassociated a strain of about −0.21% (−0.15 GPa/70GPa) for buffer and ofabout 0.057% (0.040 GPa/70 GPa) for core. The negative sign indicatesthat the strain is outward.

[0109] This means that the buffer portion of a 5.0 μm wide deep-etchedwaveguide not bonded to the underlying silicon wafer and not bonded tothe core portion of the same deep-etched waveguide would laterallyexpand by about 0.011 μm (0.21% of 5 μm) and that the buffer portion ofa 1150 μm wide deep-etched grating not bonded to the underlying siliconwafer and not bonded to the core portion of the same deep-etched gratingwould laterally expand by about 2.46 μm (0.21% of 1150 μm). Similarlythe core portion of the 5.0 μm wide deep-etched waveguide not bonded tothe underlying buffer portion of the same deep-etched waveguide wouldlaterally expand by about 0.0029 μm (0.057% of 5 μm) and that the coreportion of a 1150 μm wide deep-etched grating not bonded to theunderlying buffer portion of the same deep-etched grating wouldlaterally expand by about 0.66 μm (0.057% of 1150 μm).

[0110] In reality, since the buffer is bonded to the underlying siliconwafer and to the upper core at the buffer/core interface, the effect ofthe outward strain of buffer and of the inward strain of core would beobserved as a noticeably different sloped side-wall for a narrowwaveguide and for a wide grating.

[0111] If we assume a 2.0 μm deep-etched buffer and a 5.0 μm deep-etchcore than the single-sided strain of the upper core surface of the 5.0μm wide deep-etched waveguide and of the 1150 μm wide deep-etchedgrating could be as high as 0.0070 μm (50% of (0.011+0.0029 μm)) and1.56 μm (50% of (2.46+0.66 μm)) respectively with respect to the bottomof the resulting 7.0 μm deep-etch optical element. The expected 89.9°(90°-arctan(0.0070 μm/7.0 μm)) side-wall slope of the deep-etchedwaveguide would not be noticeable on a SEM picture but the expected77.4° (90°-arctan(1.56 μm/7.0μtm)) side-wall slope of the deep-etchedgrating would certainly be easy to see on a SEM picture.

[0112]FIG. 13 shows four SEM pictures. The first two SEM pictures showthe relative position between an isolated 5.0 μm wide deep-etchedwaveguide and its neighboring 1150 μm wide deep-etched grating at twodifferent magnifications. The third SEM picture confirms that side-wallof the 5.0 μm wide deep-etched waveguide facing the neighboring gratinghas a slope of about 90°. The fourth SEM picture confirms that side-wallof the 1150 μm wide deep-etched grating facing the neighboringdeep-etched waveguide has a much smaller slope of about 84°, slightlylarger than the expected 77.4° slope. The difference between themeasured and expected values will be discussed below.

[0113] The mechanical stress of buffer and core must be minimized as tomaintain the ideal verticality of the side-wall of the waveguides, ofthe grating and of the other integrated optical elements of the opticaldevice and allow minimum power loss from undesirable reflection andrefraction of the infrared optical beams at the side-wall of theseoptical elements.

[0114]FIG. 14 shows a graphical representation of the variable intensityshear stress building at the (compressive stress buffer)/(tensile stresscore) interface and at the (tensile stress core)/ (compressive stressclad) interface during the stress hysteresis cycle of FIG. 10 and duringthe various thermal treatments in a nitrogen ambient.

[0115] If the bonding of the buffer/core interface or of thecore/cladding interface is strong enough, the exposure of the variousoptical elements to the various thermal treatments in a nitrogen ambientcan result in a modification of the micro-structure near theseinterfaces.

[0116]FIG. 14 also shows some SEM pictures demonstrating the inducedmodification of the microstructure of core near these buffer/core andcore/cladding interfaces.

[0117]FIG. 15 shows a graphical representation of the variable intensityshear stress building at the (compressive stress buffer)/(tensile stresscore) interface and at the (tensile stress core)/(compressive stressclad) interface during the stress hysteresis cycle of FIG. 10 and duringthe various thermal treatments in a nitrogen ambient. In this case, theintensity of the shear stress is such that it results in the formationof micro-voids in core and near the interfaces as an interfacial stressrelief mechanism. These micro-voids are delineated during waferpreparation for SEM using a very light acid dip etch before loading inthe electronic microscope.

[0118] If the bonding of the buffer/core interface or of the core/cladinterface is strong enough, the exposure of the various optical elementsto the various thermal treatments in a nitrogen ambient can result insuch a modification of the micro-structure near these interfaces thatmicro-voids are forming in core and near these interfaces.

[0119]FIG. 15 also shows some SEM pictures demonstrating that theinduced modification of the microstructure of core near thesebuffer/core and core/cladding interfaces is cause the formation ofmicro-voids. It is clear on these SEM pictures that the micro-voids aregenerated and aligned horizontally in a plane about 0.5 μm away from thebuffer/bore interface. This is not that surprising since the transitionfrom the (compressive stress buffer) to the (tensile stress core) is notabsolutely abrupt at the interface and since micro-voids cannot form ina material under compressive stress.

[0120]FIG. 16 shows some SEM pictures which demonstrate the stressrelief of the variable intensity shear stress building at the(compressive stress buffer)/(tensile stress core) interface and at the(tensile stress core)/(compressive stress cladding) interface during thestress hysteresis cycle of FIG. 10, during the various thermaltreatments in a nitrogen ambient or simply during wafer cleavage for SEMpictures.

[0121] In this case, the bonding of the buffer/core interface (orcore/cladding interface) is no longer strong enough and the corepartially slips on the buffer at the buffer/core interface (or claddingpartially delaminate from core at the core/cladding interface).

[0122] In one particular case, the interface separation is only observedbetween core and buffer, indicating that core contraction is the rootcause of the delamination.

[0123] The second SEM picture of FIG. 16 shows the contraction of the1150 μm wide grating. It is clear from this picture that a portion ofcore has slipped aside over buffer and over a distance of about 0.40 μmat the periphery of the grating. This is in line with the uppercalculated contraction of 0.66 μm. The slip is again initiated from apoint located at the tip of the seam of the cladding and slightly awayfrom the buffer/core interface from which a crack did propagatehorizontally in core and about 0.5 μm away from the buffer/coreinterface. Since a crack cannot propagate in a material undercompressive stress, this crack propagation did require core to be intensile stress. Since the transition from the compressive stress bufferto the tensile stress core is not absolutely abrupt at the interface, itis normal to see the crack initiation slightly away from the buffer/coreinterface. The tensile stress-relief mechanism of core has partiallyreleases its energy by propagating a 0.40 μm long crack in the core andby allowing its side-wall to slip by 0.40 μm. This lateral of coreexplains the difference between the observed 84° of FIG. 13 and theexpected 77° from the upper calculation of the expected side-wall slope.

[0124]FIG. 17 and FIG. 18 are re-plots from J. A. Stratton,‘Electromagnetic Theory’, Chapter 9, McGraw-Hill Book Company, New York,1941.

[0125]FIG. 17 and FIG. 18 show the associated optical effect of theincidence angle of infrared light at the air/core interface on thereflection and transmission of infrared optical power (case where theinfrared light is incoming respectively from the air side and from thecore side of the side-wall of core a waveguide, a grating or of ananother optical element). It is clear from FIG. 17 and FIG. 18 that astress-induced variation of the side-wall slope from 90° to 87°, 84° orto the expected 77° will have a catastrophic effect on the loss oftransmitted power of infrared light respectively propagating in Air orin core into the air/core interface of the tip of a waveguide, into theAir/core interface of the grating or into the air/core interface ofother optical elements. It is clear from FIG. 17 and FIG. 18 that thisstress-induced loss of power will be different for the two propagatingmodes states of light (i.e. TE and TM) and thus that an undesirablepolarization dependent power loss effect (i.e. birefringence effect) isexpected.

[0126] It will be observed therefore that the mechanical stresses ofcore, buffer and cladding play a key role in the side-wall slope ofdeep-etched optical elements. It is also clearly demonstrated that thethermal treatment for 180 minutes in a nitrogen ambient at a reducedtemperature of 800° C. is associated with some residual stress-inducedmechanical problems of deep-etched optical elements (mechanical movementof sidewalls), and some residual stress-induced mechanical problems atthe buffer/core or core/cladding interfaces (micro-structural defects,micro-voiding and separation) and some residual stress-induced opticalproblems (polarisation dependant power loss). An optimisation of thethermal treatments which allows the optical properties to be maintainedwhile modifying the mechanical stress of the core is very important inthe manufacture of such integrated optical elements.

EXAMPLE

[0127] The technique in accordance with the preferred embodiment of theinvention allows the simultaneous optimization of the optical and of themechanical properties of buffer (cladding) and core in aseven-dimensional space. This consists of a first independent variable,the SiH₄ flow, fixed at 0.20 std litre/min; a second independentvariable, the N₂O flow, fixed at 6.00 std litre/min; a third independentvariable, the N₂ flow, fixed at 3.15 std litre/min; a fourth independentvariable, the PH₃ flow, fixed at 0.50 std litre/min; a fifth independentvariable, the total deposition pressure, fixed at 2.60 Torr; and a sixthindependent variable, the post-deposition thermal treatment being variedas follows:

[0128] 30 minutes duration thermal treatment in a nitrogen ambient at600° C.;

[0129] 30 minutes duration thermal treatment in a nitrogen ambient at700° C.;

[0130] 30 minutes duration thermal treatment in a nitrogen ambient at750° C.;

[0131] 30 minutes duration thermal treatment in a nitrogen ambient at800° C.;

[0132] 30 minutes duration thermal treatment in a nitrogen ambient at850° C.;

[0133] 30 minutes duration thermal treatment in a nitrogen ambient at900° C.

[0134] A seventh dimension is the observed FTIR characteristics ofvarious buffer (cladding) and core silica-based optical elements, asreported in: FIG. 3d, FIG. 4d, FIG. 5d, FIG. 6d, FIG. 7d, FIG. 8d, &FIG. 9d:

[0135]FIG. 3d, FIG. 4d, FIG. 5d, FIG. 6d, FIG. 7d, FIG. 8d and FIG. 9dshow the FTIR spectra of PECVD silica films deposited using acommercially available PECVD system, the “Concept One” systemmanufactured by Novellus Systems in California, USA, using the fixedoptimum total deposition pressure and the fixed flow rates of silane(SiH₄), of nitrous oxide (N₂O), of nitrogen (N₂), and of phosphine (PH₃)as described in our co-pending U.S. patent application Ser. No.09/799,491. These spectra are obtained after 30 minutes thermaltreatments in a nitrogen ambient at various temperatures in a standarddiffusion tube. It is clear that this new patent application describes away to independently optimize the thermal treatment and the opticalproperties of buffer, core and cladding as to allow the thermaltreatment optimization of the mechanical properties of the silica-basedoptical elements without any interaction with the optical propertiesstable of these optical elements:

[0136]FIG. 3d shows that the intense and small FWHM Si—O—Si “rockingmode” absorption peak (centred at 460 cm⁻¹) and Si—O—Si“in-phase-stretching mode” absorption peak (centred at 1080 cm⁻¹) of thefixed deposition pressure of 2.60 Torr of FIG. 3b and of the fixed PH₃flow rate of 0.50 std litre/min of the FIG. 3c is maintained as thetemperature of the 30 minutes thermal treatments in a nitrogen ambientis gradually decreased from 900° C. to 600° C. This means thatindependently of the SiH₄ gas flow of the N₂O gas flow of the N₂ gasflow and of the PH₃ gas flow and as long as the deposition pressure isfixed to 2.60 Torr, the basic FTIR spectra of silica-based opticalcomponents are not affected by the temperature variation (between 600°C. and 900° C.) of the 30 minutes thermal treatment in a nitrogenambient;

[0137]FIG. 4d shows that the elimination of the Si—OH oscillators(centered at 885 cm⁻¹) of the fixed deposition pressure of 2.60 Torr ofFIG. 4b and of the fixed PH₃ flow rate of 0.50 std litre/min of the FIG.4c is maintained. FIG. 4d also shows that the elimination of the Si—ONoscillators (centred at 950 cm⁻¹) of the fixed deposition pressure of2.60 Torr of FIG. 4b and of the fixed PH₃ flow rate of 0.50 stdlitre/min of the FIG. 4c is also maintained. This very spectacularimproved elimination of the residual Si—ON oscillators after a 30minutes thermal treatment of only 600° C. contrasts with theupper-mentioned results of typical PECVD silica films of FIG. 4a whichstill incorporate a lot of Si—ON oscillators even after a 180 minutesthermal treatment in a nitrogen ambient at a much higher temperature of1100 ° C. This also contrasts with the upper-mentioned results of PECVDbuffer (cladding) deposited at a non-optimized pressure of less than2.40 Torr by our co-pending U.S. patent application Ser. No. 09/833,711of FIG. 4b which still incorporate a large number of Si—ON oscillatorseven after a 180 minutes thermal treatment in a nitrogen ambient at amuch higher temperature of 800° C. The optimum separation and deepvalley between the Si-O-Si “in-phase-stretching mode” absorption peak(1080 cm⁻¹) and the Si—O—Si “bending mode” absorption peak (810 cm⁻¹) ofthe fixed deposition pressure of 2.60 Torr of FIG. 4b and of the fixedPH₃ flow rate of 0.50 std litre/min of the FIG. 4c is also maintained.This means that this new technique allows the elimination of the Si—OHoscillators and of the the Si—ON oscillators independently of thethermal treatment of buffer, core and cladding as to allow the thermaltreatment optimization of the mechanical properties of the silica-basedoptical elements without any interaction with the Si—OH oscillators andof the the Si—ON oscillators of these optical elements.

[0138]FIG. 5d shows the gradual appearance of the P═O oscillators(centered at 1330 cm⁻¹ and which does not have a higher harmonics whichcould cause optical absorption in the 1.30 to 1.55 μm optical bands) asthe temperature of the 30 minutes thermal treatment in a nitrogenambient is increased from 600° C. to 900° C.

[0139]FIG. 6d shows that the elimination of the N═N oscillators(centered at 1555 cm⁻¹) of the fixed deposition pressure of 2.60 Torr ofFIG. 6b and of the fixed PH₃ flow rate of 0.50 std litre/min of the FIG.6c is maintained. This also contrasts with the upper-mentioned resultsof typical PECVD silica films of FIG. 6a which require a 180 minutesthermal treatment in a nitrogen ambient at a temperature of 1000° C. inorder to achieve similar results. This also contrasts with theupper-mentioned results of PECVD buffer (cladding) deposited at anon-optimized pressure of less than 2.40 Torr in our co-pending U.S.patent application Ser. No. 09/833,711 of FIG. 6b which stillincorporate a large number of N═N oscillators even after a 180 minutesthermal treatment in a nitrogen ambient at a much higher temperature of800° C. This means that this new technique allows the elimination of theN═N oscillators independently of the thermal treatment of buffer, coreand cladding as to allow the thermal treatment optimization of themechanical properties of the silica-based optical elements without anyinteraction with the N═N oscillators of these optical elements.

[0140]FIG. 7d shows that the Si═O oscillators (centered at 1875 cm⁻¹)and the unknown oscillator (centered at 2010 cm⁻¹) of the fixeddeposition pressure of 2.60 Torr of FIG. 7b and of the fixed PH₃ flowrate of 0.50 std litre/min of the FIG. 7c are unchanged. These effectsare not that important since only the fourth harmonics of the Si═Ooscillators could absorb in the 1.30 to 1.55 μm optical bands;

[0141]FIG. 8d shows that the elimination of the Si—H oscillators(centered at 2260 cm⁻¹ and which third harmonics could cause an opticalabsorption between 1.443 and 1.508 μm) of the fixed deposition pressureof 2.60 Torr of FIG. 8b and of the fixed PH₃ flow rate of 0.50 stdlitre/min of the FIG. 8c is maintained. This means that this newtechnique allows the elimination of the Si—H oscillators independentlyof the thermal treatment of buffer, core and cladding as to allow thethermal treatment optimization of the mechanical properties of thesilica-based optical elements without any interaction with the Si—Hoscillators of these optical elements.

[0142]FIG. 9d shows that the spectacular complete elimination of theSi:N—H oscillators (centered at 3380 cm⁻¹ whose 2^(nd) harmonics couldcause an optical absorption between 1.445 and 1.515 μm) of the fixeddeposition pressure of 2.60 Torr of FIG. 9b and of the fixed PH₃ flowrate of 0.50 std litre/min of the FIG. 9c is maintained. This contrastswith the upper-mentioned results of typical PECVD silica films of FIG.9a which require a 180 minutes thermal treatment in a nitrogen ambientat a temperature of 1100° C. in order to achieve similar results. Thisalso contrasts with the upper-mentioned results of PECVD buffer(cladding) deposited at a non-optimized pressure of less than 2.40 Torrin our co-pending U.S. patent application Ser. No. 09/833,711 of FIG. 9bwhich still incorporate a lot of Si:N—H oscillators even after a 180minutes thermal treatment in a nitrogen ambient at a much highertemperature of 800° C. FIG. 9d shows that the a spectacular completeelimination of the SiN—H oscillators (centered at 3420 cm⁻¹ whose 2^(nd)harmonics could cause an optical absorption between 1.445 and 1.479 μm)of the fixed deposition pressure of 2.60 Torr of FIG. 9b and of thefixed PH₃ flow rate of 0.50 std litre/min of the FIG. 9c is alsomaintained. This contrasts with the upper-mentioned results of typicalPECVD silica films of FIG. 9a which require a thermal treatment in anitrogen ambient at a temperature of 1000° C. in order to achievesimilar results. This also contrasts with the upper-mentioned results ofPECVD buffer (cladding) deposited at a non-optimized pressure of lessthan 2.40 Torr in our co-pending U.S. patent application Ser. No.09/833,711 of FIG. 9b which still incorporate a large number of Si:N—Hoscillators even after a 180 minutes thermal treatment in a nitrogenambient at a much higher temperature of 800° C. FIG. 9d also shows thatthe complete elimination of the SiO—H oscillators (centered at 3510 cm⁻¹whose 2^(nd) harmonics could cause an optical absorption between 1.408and 1.441 μm) of the fixed deposition pressure of 2.60 Torr of FIG. 9band of the fixed PH₃ flow rate of 0.50 std litre/min of the FIG. 9c ismaintained. This contrasts with the upper-mentioned results of typicalPECVD silica films which require a thermal treatment in a nitrogenambient at a temperature of 900° C. in order to achieve similar results.Finally, FIG. 9d also shows that the complete elimination of the HO—Hoscillators (centered at 3650 cm⁻¹ whose ₂nd harmonics could cause anoptical absorption between 1.333 and 1.408 μm) of the fixed depositionpressure of 2.60 Torr of FIG. 9b and of the fixed PH₃ flow rate of 0.50std litre/min of the FIG. 9c is maintained. This means that this newtechnique allows the elimination of the Si:N—H oscillators, of the SiN—Hoscillators, of the SiO—H oscillators and of the HO—H oscillatorsindependently of the thermal treatment of buffer, core and cladding asto allow the thermal treatment optimization of the mechanical propertiesof the silica-based optical elements without any interaction with theSi:N—H oscillators, with the SiN—H oscillators, with the SiO—Hoscillators or with the HO—H oscillators of these optical elements. Itis clear from these various FTIR spectra that this new technique allowsthe elimination of the various thermally-induced and stress-relatedresidual mechanical problems by the optimisation of the thermaltreatment (i.e. the use of the Regions B 1, B2, B3, C1 and C2 of thestress hysteresis of FIG. 10) without affecting the optical absorptionproperties of optical elements in the 1.55 μm wavelength (and/or 1.30wavelength) optical region.

[0143] It is then clear from these various FTIR spectra, from the stresshysteresis of buffer, core and cladding and from the various presentedSEM pictures that this new technique is key to achieving the required‘delta-n’ while eliminating the undesirable residual Si:N—H oscillators(observed as a FTIR peak centered at 3380 cm⁻¹ whose 2^(nd) harmonicscould cause an optical absorption between 1.445 and 1.515 μm), SiN—Hoscillators (centered at 3420 cm⁻¹ whose 2^(nd) harmonics could cause anoptical absorption between 1.445 and 1.479 μm) and SiO—H oscillators(centered at 3510 cm⁻¹ and whose 2^(nd) harmonics could cause an opticalabsorption between 1.408 and 1.441 μm) after an optimised thermaltreatment in a nitrogen ambient which can provide improved silica-basedoptical elements with reduced optical absorption in the 1.55 μmwavelength (and/or 1.30 wavelength) optical region without the residualstress-induced mechanical problems of deep-etched optical elements(mechanical movement of side-walls), without the residual stress-inducedmechanical problems at the buffer/core or core/cladding interfaces(micro-structural defects, micro-voiding and separation) and without theresidual stress-induced optical problems (polarisation dependant powerloss).

[0144] It will be apparent to one skilled in the art that manyvariations of the invention are possible. The PECVD silica films couldbe deposited at a temperature different than 400° C. It could bedeposited at any temperature between 100 and 650° C.

[0145] The PECVD equipment could be different than the Novellus ConceptOne. The requirement is to provide independent control of the four basiccontrol parameters: SiH₄ gas flow rate, N₂O gas flow rate, N₂ gas flowrate and total deposition pressure.

[0146] The buffer (cladding) local optimum (SiH₄ gas flow of 0.20 stdlitre/min, N₂O gas flow of 6.00 std litre/min, N₂ gas flow of 3.15 stdlitre/min and a total deposition pressure of 2.60 Torr) is thisfour-independent-variables space could have a different set ofcoordinates (SiH₄, N₂O, N₂, deposition pressure) using the same NovellusConcept One equipment.

[0147] The buffer (cladding) local optimum could have a different set ofcoordinates (SiH₄, N₂O, N₂, deposition pressure) in another PECVDequipment.

[0148] The core local optimum (SiH₄ gas flow of 0.20 std litre/min, N₂Ogas flow of 6.00 std litre/min, N₂ gas flow of 3.15 std litre/min, PH₃gas flow of 0.57 std litre/min, and a total deposition pressure of 2.60Torr) is this five-independent-variables space could have a differentset of coordinates (SiH₄, N₂O, N₂, PH₃, deposition pressure) using thesame Novellus Concept One equipment.

[0149] The core local optimum could have a different set of coordinates(SiH₄, N₂O, N₂, PH₃, deposition pressure) in another PECVD equipment.The ‘delta-n’ could be different than 0.015 and range between 0.005 and0.020.

[0150] The SiH₄ silicon raw material gas could be replaced by analternate silicon containing gas, such as: silicon tetra-chloride,SiCl₄, silicon tetra-fluoride, SiF₄, disilane, Si₂H₆, dichloro-silane,SiH₂Cl₂, chloro-fluoro-silane SiCl₂F₂, difluoro-silane, SiH₂F₂ or anyother silicon containing gases involving the use of hydrogen, H,chlorine, Cl, fluorine, F, bromine, Br, and iodine, I.

[0151] The N₂O oxidation gas could be replaced by an alternate oxygencontaining gas, such as: oxygen, O₂, nitric oxide, NO₂, water, H₂O,hydrogen peroxide, H₂O₂, carbon monoxide, CO or carbon dioxide, CO₂.

[0152] The N₂ carrier gas could be replaced by an alternate carrier gas,such as: helium, He, neon, Ne, argon, Ar or krypton, Kr.

[0153] The PH₃ doping gas could be replaced by an alternate gas, suchas: diborane, B₂H₆, Arsine (AsH₃), Titanium hydride, TiH₄ or germane,GeH₄, Silicon Tetrafluoride, SiF₄ of carbon tetrafluoride, CF₄.

[0154] The high temperature thermal treatment in nitrogen can beperformed at a temperature different than 800° C. The preferred range isfrom 400 to 1200° C.

[0155] The high temperature thermal treatment can be performed in adifferent ambient than nitrogen. Other ambient gases or mixtures ofgases may include oxygen, O₂, hydrogen, H₂, water vapour, H₂O, argon,Ar, fluorine, F₂, carbon tetrafluoride, CF₄, nitrogen trifluoride, NF₃,hydrogen peroxide, H₂O₂.

[0156] The optical region of interest is not limited to the 1.30 to 1.55μm optical region since the higher oscillation harmonics of theeliminated oscillators have other optical benefits at longer or shorterwavelengths. The wavelengths of the first, second, third and fourthharmonics of these oscillators are to be covered by this patent.

[0157] The invention has application in may devices other than Mux orDmux devices. The following is a list of suitable devices, which is notintended to be exhaustive: Add-After-Drop Filters (AADF) devices;Arrayed Wave Guide (AWG) and Arrayed Wave Guide Grating (AWGG) devices;thermal Arrayed Wave Guide (AAWGG) devices; Charged Coupled Devices(CCD) devices; Distributed Feedback Laser Diode (DFB-LD) devices; ErbiumDoped Fiber Amplifier (EDFA) devices; Fiber-To-The-Home (FTTH)application devices; Four Wave Mixing (FWM) devices; Fresnel Mirror (FM)devices; Laser Diode (LD) devices; Light Emitting Diodes (LED) devices;Mach-Zenhder (MZ), Mach-Zenhder Interferometer (MZI), Mach-ZenhderInterferometer Multiplexer (MZIM) devices; Micro-Opto-Electro-MechanicalSystems (MOEMS) devices; Monitor Photo Diode (MPD) devices;Multi-Wavelength Optical Sources (MWOS) devices; Optical Add/DropMultiplexers (OADM) devices; Optical Amplifier (AF) devices; OpticalCross-Connect (OCC, OXC) devices; Optical Cross Point (OCP) devices;Optical Filter (OF) devices; Optical Interferometer (OI) devices;Optical Network Unit (ONU) devices; Optical Saw Wave (OSW) devices;Optical Splitter (OS) devices; Optical Switch (OSW) and Optical SwitchModule (OSM) devices; Photonic ATM (PATM) switching devices; PlanarLightwave Circuits (PLC) devices; Positive Emitter Coupled Logic (PECL)devices; Quarter Wave (QW) devices; Receiver Photo Diode (RPD) devices;is Semiconductor Optical Amplifier (SOA) devices; Spot-Size converterintegrated Laser Diode (SS-LD) devices; Sub-Carrier Multiplexing OpticalNetwork Unit (SCM-ONU) devices; Temperature Insensitive Arrayed WaveGuide (TI-AWG) devices; Thermo-Optic (TO) devices and Thermo-OpticSwitch (TOS) devices; Time Compression Multiplexing—Time DivisionMultiple Access (TCM-TDMA) devices; Time Division Multiplexing (TDM)devices; Tunable Receiver (TR) devices; Uniform-Loss Cyclic-FrequencyArrayed Wave Guide (ULCF-AWG) devices; Vertical Cavity Surface EmittingLaser (VCSEL) devices; Wavelength Dispersive Multiplexing (WDM),Wavelength Dispersive Multiplexing Transceivers (WDMT) devices;Micro-Electro-Mechanical Systems (MEMS) device: Information TechnologiesMEMS devices; Medical/Biochemical MEMS devices: Biochip devices;Lab-On-A-Chip (LOAC) devices; Micro-Total Analysis System (μ-TAS)devices; Automotive MEMS devices; Industrial/Automation MEMS devices;Environmental Monitoring MEMS devices; Telecommunications MEMS devices.

[0158] Although the invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example and is not to be taken by way of limitation, the spirit andscope of the invention being limited only by the terms of the appendedclaims.

We claim:
 1. A method of depositing an optical quality silica film byPECVD (Plasma Enhanced Chemical Vapor Deposition), comprising: a)independently setting a predetermined flow rate for a raw material gas;b) independently setting a predetermined flow rate for an oxidation gas;c) independently setting a predetermined flow rate for a carrier gas; d)independently setting a predetermined total deposition pressure; and e)applying a post deposition heat treatment to the deposited film at atemperature selected to optimize the mechanical properties withoutaffecting the optical properties determined in steps a to d.
 2. A methodas claimed in claim 1, further comprising independently setting apredetermined flow rate for a dopant gas.
 3. A method as claimed inclaim 2, wherein the observed FTIR characteristics of the deposited filmare monitored to determine the optimum post deposition heat treatmenttemperature.
 4. A method as claimed in claim 1, wherein the postdeposition heat treatment temperature lies in the range 600 to 900° C.5. A method as claimed in claim 4, wherein the deposition is carried outat a temperature in the range 100 to 650° C.
 6. A method as claimed inclaim 5, wherein the deposition is carried out at a temperature of about400° C.
 7. A method as claimed in claim 1, wherein the raw material gasis selected from the group consisting: silane, SiH₄; silicontetra-chloride, SiCl₄; silicon tetra-fluoride, SiF₄; disilane, Si₂H₆;dichloro-silane, SiH₂Cl₂; chloro-fluoro-silane SiCl₂F₂; difluoro-silane,SiH₂F₂; and any other silicon containing gas containing hydrogen, H,chlorine, Cl, fluorine, F, bromine, Br, or iodine, I.
 8. A method asclaimed in claim 7, wherein the oxidation gas is selected from the groupconsisting of: nitrous oxide, N₂O; O₂, nitric oxide, NO₂; water, H₂O;hydrogen peroxide, H₂O₂; carbon monoxide, CO; and carbon dioxide, CO₂ 9.A method as claimed in claim 8, wherein the carried gas is selected fromthe group consisting of nitrogen, N₂; helium, He; neon, Ne; argon, Ar;or krypton, Kr.
 10. A method as claimed in claim 2, wherein the dopantgas is selected from the group consisting of phosphene, PH₃; diborane,B₂H₆; Arsine (AsH₃); Titanium hydride, TiH₄; germane, GeH₄; SiliconTetrafluoride, SiF₄; and carbon tetrafluoride, CF₄.
 11. A method asclaimed in claim 2, wherein the raw material gas is SiH₄, the oxidationgas is N₂O, the carrier gas is N₂, and the dopant gas is PH₃.
 12. Amethod as claimed in claim 11, wherein the SIH₄ gas flow is set at about0.2 std liters/min., the N₂O gas flow is set at about 6.00 stdliters/min., the N2 flow is set at about 3.15 liters/min., and the PH₃is set at about 0.50 std liters/min.
 13. A method of depositing anoptical quality silica film by PECVD (Plasma Enhanced Chemical VaporDeposition), comprising: a) independently setting a flow rate for SiH₄at about 0.2 std liters/min.; b) independently setting a flow rate forN2O at about 6.00 0.2 std liters/min.; c) independently setting a flowrate for a carrier gas; d) independently setting a predetermined totaldeposition pressure; and e) applying a post deposition beat treatment tothe deposited film at a temperature between 600° and 900° C selected tooptimize the mechanical properties without affecting the opticalproperties determined in steps a to d.
 14. A method as claimed in claim13, wherein the carrier gas is N₂ and the flow rate is set at about 3.152 std liters/min.
 15. A method as claimed in claim 14, furthercomprising independently setting a predetermined flow rate for a dopantgas.
 16. A method as claimed in claim 15, wherein the dopant gas is PH₃and the flow rate is set at about 0.50 std liters/min.
 17. A method asclaimed in claim 15, wherein the total deposition pressure is set atabout 2.6 Torr.
 18. A method as claimed in claim 13, wherein theobserved FTIR characteristics of the deposited film are monitored todetermine the optimum post deposition heat treatment temperature.
 19. Amethod as claimed in claim 13, wherein said deposited film forms abuffer, core or cladding of an optical component.
 20. A method asclaimed in claim 19, wherein said optical component is a multiplexer ordemultiplexer.